The invention relates to a process for separating calcium nitrate tetrahydrate by crystallization from a liquor obtained by digesting phosphate rock with nitric acid.
Such a process is common knowledge and constitutes an essential part of the so-called Odda nitrophosphate process (see for instance British Patent No. 339,340). In this known process the calcium nitrate formed in the digestion is continuously separated off by crystallization as tetrahydrate by cooling the digestion liquor by means of a cold cooling liquid and separating the crystallized salt from the mother liquor, which is processed, to form a mixed fertilizer. Depending on the final temperature of the cooled digestion liquor, about 60-90% of the calcium nitrate present in the digestion liquor will crystallize out in said process.
A disadvantage of this known process is that the crystals formed are very nonhomogeneous in terms of particle size and, moreover, have a relatively small average particle diameter. Consequently, it is very difficult indeed for the crystals to be separated off. For instance in the separation of the crystals from the mother liquor by centrifugation or filtration frequently results in a substantial slip-through of crystals. The substantial-slip through of crystals is disadvantageous because during subsequent processing of the mother liquor into mixed fertilizer products water-insoluble and sometimes citrate-insoluble phosphates are formed. The formation of such insoluble materials results in a loss of available nutrient phosphate.
It is true that a larger quantity of calcium nitrate in the digestion liquor may be crystallized by cooling the latter to very low temperatures as described in, for instance, Proces Techniek, dated 2.5.1969, pages 312-316. The disadvantage of this process is that the cooling to very low temperatures requires a large amount of energy and a very high cooling capacity. Moreover, it has been found that, in this process, too, the calcium nitrate crystals formed have a rather small and inhomogeneous particle size, so that in the separation of these there will be a certain degree of slip-through of these crystals. The two processes described above consequently require substantial investments, either for effectively separating off the crystals or for extra cooling capacity, in order to obtain a mother liquor having a sufficiently low calcium nitrate concentration. These known processes, moreover, have the disadvantage that during the process the calcium nitrate tetrahydrate formed on the cooling surfaces must be removed. This involves an extra increase in the amount of cooling energy required.
U.S. Pat. No. 2,813,777 discloses a batchwise process for crystallizing out calcium nitrate from nitric acid digestion liquors. In this process the calcium nitrate crystallizes out mainly in the form of relatively coarse crystals (0.5-1.0 mm), after which these crystals can be separated off from the mother liquor by filtration. This is achieved by adding calcium nitrate-containing seed material to a digestion liquor having a temperature 2.degree.-10.degree. C. lower than the saturation temperature and by further cooling the liquor obtained. In this process the crystals obtained show a fairly wide particle size distribution. Although the crystals can largely be separated off through filtration, it has been found that, long filtration times are necessary because of the non-homogeneous particle sizes, i.e., the wide particle size distribution. In this process, too, there will be a growth of fine calcium nitrate tetrahydrate material on the cooling surfaces.